1. Field of the Invention
The present invention relates to a PZT piezoelectric film and a method for manufacturing the same, a piezoelectric device using the piezoelectric film, and a liquid discharge apparatus.
2. Description of the Related Art
A piezoelectric device which includes a piezoelectric film having piezoelectricity, in which the film stretches or contracts according to the intensity of the applied electric field, and electrodes for applying an electric field to the film is used as an actuator mounted on an inkjet recording head and the like. As for piezoelectric materials, PZT (lead zirconate titanate) and substituted systems of PZT in which a part of A-site and/or B-site is substituted by another element are known. Hereinafter, PZT and substituted systems of PZT are collectively referred to as “PZT system”.
It is known that PZT doped with a donor ion having a higher valence number than that of the replaced ion has a higher piezoelectric performance than pure PZT. As donor ions for substituting B-site Zr4+ and/or Ti4+, V5+, Nb5+, Ta5+, Sb5+, Mo6+, W6+, and the like are known. A PZT system perovskite oxide in which a part of B-site is substituted by another element M may be represented by General Expression (P) below.Pba(Zrx,Tiy,Mb-x-y)bOc  (P)
(where, M represents one or more types of B-site elements,
0<x<b, 0<y<b, 0≦b-x-y,
a:b:c=1:1:3 is standard, but the standard molar ratio may deviate within a range in which a perovskite structure can be obtained.)
In the application for the inkjet recording head and the like, it is preferable that the piezoelectric film has a high piezoelectric constant, and a high piezoelectric performance, for example, with piezoelectric constant d31≧150 pm/V is demanded. Also, for the inkjet recording head, a low replacement frequency of the head is desirable, and hence sufficient durability is demanded for the piezoelectric film in practical use.
For a PZT system perovskite oxide, shortage of Pb causes a pyrochlore phase to be formed and the piezoelectric performance is degraded. Therefore, an A-site rich composition, in which the amount of Pb of an A-site element is greater than the stoichiometric proportion, is regarded to be desirable. The insulation resistance and durability, however, tend to be degraded when the amount of Pb is increased.
In this respect, reference is made to literatures, “Effect of Stoichiometry on the Dielectric Properties and Life Performance of (Pb0.875Ba0.125)[(Mg1/3Nb2/3)0.5(Zn1/3Nb2/3)0.3Ti0.2]O3 Relaxor Dielectric Ceramic: Part I, Dielectric Properties”, H. Kanai et al., J. Am. Ceram. Soc., Vol. 76, No. 2, pp. 454-458, 1993 and “Effect of Stoichiometry on the Dielectric Properties and Life Performance of (Pb0.875Ba0.125)[(Mg1/3Nb2/3)0.5(Zn1/3Nb2/3)0.3Ti0.2]O3 Relaxor Dielectric Ceramic: Part II, Life Performance”, H. Kanai et al., J. Am. Ceram. Soc., Vol. 76, No. 2, pp. 459-464, 1993, which are about a PZT system dielectric body having a high dielectric constant for a laminated capacitor. The literatures describe that shortage of Pb causes a pyrochlore phase to be formed, but the insulation resistance and durability are improved, so that the high durability and high capacitance can be achieved. For dielectric applications, i.e., applications in which only capacitance is important, the presence of certain amount of pyrochlore phase does not have any impact on use, thus the presence of pyrochlore phase does not pose any big problem.
For piezoelectric applications, however, presence of any pyrochlore phase largely degrades the piezoelectric performance. Further, for piezoelectric applications in which a piezoelectric film having a larger film thickness is used in comparison with that for dielectric applications, the film having pyrochlore phase poses a problem that a crack is likely to occur or the film is likely to be detached from the substrate. Therefore, a piezoelectric film with little amount of Pb and having pyrochlore phase can not be employed in practical use in piezoelectric applications.
U.S. Pat. No. 7,348,715 (Patent Document 1) describes that the durability of a PZT system is improved when the molar ratio a/b between A-site element and B-site element is in the range from 0.85 to less than 1.0 (claim 1). Patent Document 1 does not mention about pyrochlore phase. But, the film is formed by an ordinary vapor phase film forming method (RF magnetron sputtering method) and a specific scheme for preventing formation of pyrochlore phase is not taken. Therefore, formation of pyrochlore phase is unavoidable under the conditions with little amount of Pb described in Patent Document 1.
As described above, the piezoelectric performance and durability are incompatible properties for PZT system piezoelectric films, and the compatibility between them has not been achieved yet.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a PZT system piezoelectric film having both excellent piezoelectric performance and durability, and a method for manufacturing the same.